Research and development is being actively performed on organic electroluminescence devices (hereinafter, also referred to as “devices” or “organic EL devices”) due to their ability to emit light with high luminance intensity at low driving voltages. An organic electroluminescence device has a pair of electrodes and an organic layer therebetween in which electrons injected from the cathode recombine with holes injected from the anode in the organic layer to create excitons whose energy is utilized for light emission.
Phosphorescent materials are currently used to achieve high efficiency of devices. Further, dope-type devices using light emitting layers in which host materials are doped with light emitting materials are widely employed.
Host materials are also being developed. For example, Patent Document 1 discloses a 1,3-bis(N-carbazolyl)benzene (mCP) derivative substituted with a phenyl group for the purpose of fabricating a device with high luminous efficiency, few pixel defects and good heat resistance. The patent document also discloses an organic electroluminescence device using the derivative as a host material.
However, there is still a need for organic electroluminescence devices whose luminous efficiency and durability are compatible with each other at a higher level than the device described in Patent Document 1.
Patent Document 2 discloses an organic EL device which includes at least one organic compound layer composed of an organic compound material having an impurity concentration below 1,000 ppm thereby improving durability of a device. However, Patent Document 2 fails to specify the kind of impurities that have a great influence on the performance of the device.
Further, Patent Document 3 discloses organic EL devices using, as host materials, various N-carbazolylbenzene derivatives having a purity of 99.3% to 99.9%, as analyzed by high-performance liquid chromatography (HPLC). However, Patent Document 3 also fails to specify the kind of impurities that have a great influence on the performance of the devices.